We estimated diet composition of sympatric Mexican spotted
(Strix occidentalis lucida, n = 7 pairs of owls) and great
horned owls (Bubo virginianus, n = 4 pairs) in ponderosa
pine (Pinus ponderosa) - Gambel oak (Quercus gambelii)
forest, northern Arizona. Both species preyed on mammals, birds,
and insects; great horned owls also ate lizards. Mammals dominated
the diet of both species. Mammals comprised 63 and 62% of all
prey items identified in diets of spotted and great horned owls,
respectively, and 94 and 95% of prey biomass. Both species primarily
preyed on a few groups of small mammals. Observed overlap in
diet composition between species (0.95) was greater than expected
based on null models of diet overlap, and the size range of prey
taken overlapped entirely. Mean prey mass was similar for both
species (great horned owl, 47.0 ± 7.4 g [SE], n
= 94 items; spotted owl, 40.1 ± 1.8 g, n = 1,125
items). Great horned owls consumed larger proportions of diurnally
active prey than spotted owls, which primarily consumed nocturnally
active mammals. Our results, coupled with a previous analysis
showing that these owls foraged in the same general areas (Ganey
and others 1997), suggests that they could compete for food resources,
which are assumed to be limiting in at least some years. They
may minimize the potential for resource competition, however,
by concentrating foraging activities in different habitats (Ganey
and others 1997) and by foraging at different times, when different
suites of prey species are active.